The present invention broadly relates to contact type electrical switch devices suitable for high cycle switching applications, and deals more particularly with a method for making thick film switch elements useful for contact switches, potentiometers and encoders.
Many low current switching applications, including electronic encoding devices, employ sliding electrical contacts, often referred to as wipers, rakes, or brushes, that cooperate with metal terminals or conductors on a planar substrate to make and break electrical circuits. These types of electrical switching devices have been long used in a variety of applications because of their high reliability and simplicity of construction. In recent years, such switching devices have found increasing use for sensing the position of a movable element relative to a reference point. For example, sensors are often used in vehicles to sense the position of an accelerator pedal forming part of an electronic throttle control (ETC) system, sometimes referred to as drive-by-wire systems. In an ETC system, the accelerator pedal is electronically, rather than mechanically, linked to the vehicle""s engine. This type of sensor, commonly known as a pedal position sensor (PPS), is mounted on the accelerator pedal such that it translates the rotational displacement of the pedal into an electrical signal that is proportional to pedal position. This signal is delivered to the engine""s ECU (electronic control unit) which in turn controls fuel delivery to the vehicle""s engine. Rotary position sensors are also mounted on engine throttle bodies to sense the actual position of the carburetor throttle plate. Like PPSs, throttle position sensors (TPSs) are subject to high cycling demands.
Rotary position sensors of the type described above are normally in the form of a potentiometric device, comprising one or more wiper contacts connected to a rotatable input shaft on the sensor which, in the case of a PPS, is driven by displacement of the pedal. The wipers slide over a conductor pattern deposited on a substrate such as polyamide, FR-4, thermoset or ceramic. The conductor may be a plated copper, polymer thick film silver or precious metal thick film. A resistor film of electronically conductive polymer is deposited over the conductor to form a variable resistor element. Precious metal contacts are positioned over the resistor element in a manner such that sliding movement of the wiper over the conductor pattern creates a variable potentiometric linear output that is proportional to rotational the position of the sensor""s input shaft, and is thus indicative of pedal position. One type of known position sensor configuration employs a flexible polymer resistor film on which resistive tracks defining a potentiometer and/or switch are formed using conventional thick film deposition techniques. While polymer resistor films may initially have acceptably low contact resistance, with mechanical cycling, such film tends to generate high resistance wear debris that contributes to high contact resistance and eventual electrical noise. This debris is created as a result of the movable contact wipers dislodging material from the surface of the substrate. The debris material is carried along with the wiper and intermittently builds up at the interface between the wiper and the substrate.
In certain position sensor applications, it is desirable to incorporate a switched or stepped (digital) output in addition to the continuous potentiometer output. These switches are typically formed simultaneously with the potentiometer resistor circuit on a common substrate, thus permanently fixing the position of the switch contacts relative to the position of the potentiometer. Such integrated switches are used to provide control signals to transmissions or to provide signals to the vehicle""s engine ECU which validate that the pedal is in either the idle or wide open throttle position. The problem of contact wear and signal noise caused by debris accumulation is particularly acute in the case of the switch circuits, in large part because the switch conductors formed on the substrate create discontinuities or steps which the contact wiper must pass over. The wiper tends to collide with the edges of the stepped conductors, increasing the likelihood that material will be dislodged from the substrate. Conductor contacts formed on, a substrate using traditional thick film deposition techniques typically create a step height of approximately 0.5 to 1.5 mils (0.0005-0.0015 inches). Even in the case of a potentiometer, the resistor that is printed on top of the thick film resistor terminations conforms to the profile of the termination below and creates a step on the resistor that the contact wiper must traverse at the mechanical end of travel. Much like the above described switch application, this step contributes to wiper contact bounce, and acts as a debris generation and accumulation site.
Although prior art position sensors of the type described above have been marginally acceptable for some vehicles applications in the past, the increasingly stringent requirements for performance and service life for future vehicle applications renders these existing sensors inadequate.
One attempted solution to the problem of contact noise and wear in high cycle switch applications is disclosed in U.S. Pat. No. 5,169,465 to Riley, issued Dec. 8, 1992. The Riley patent discloses a thick film switch element that includes a high temperature glass frit fused to a ceramic substrate. A cermet layer, typically a noble metal such as silver, having a low temperature glass matrix is fired in a conventional furnace which causes the cermet layer to sink into the glass frit layer such that the resulting thickness of the switch element layer is approximately equal to the original thickness of the glass frit layer. The exposed surface of the resulting thick film switch element product is substantially smooth, and the joint between the low temperature cermet layer and the high temperature glass frit layer is substantially uniform, i.e., flush. The thick film switch element of Riley requires tight process control over material composition and firing temperatures of both the underlying glass frit layer, and the overlying cermet layer. These more stringent process controls and the materials contribute to higher costs.
It would therefore be desirable to provide a superior sensor construction suitable for high cycle switching and encoding applications that utilizes commercially available materials and takes advantage of standard processing techniques which do not require precise control. The present invention is directed toward satisfying this need.
The present invention, in one embodiment, is directed to an electrical switch device including a switch element having a low temperature co-fired dielectric substrate, electrical conductors embedded in the substrate and having a surface substantially flush with the substrate surface, and the device including a wiper contact in sliding engagement with the electrical conductors.
The present invention, in a second embodiment, is directed to a method of manufacturing a switch element that includes providing a low temperature co-fired dielectric substrate in a green state, depositing an electrically conductive material onto a face of the substrate, pressing the conductive material into the substrate until the material is substantially flush with the substrate face, and then firing the substrate and conductive material to form the switch element.
This method embodiment of the invention has the advantages of using commercially available low temperature co-fired dielectric materials and standard thick film processing conditions. Also, because the conductive material and substrate are co-fired, only one firing step is required. These and other features and advantages of the invention will be apparent from the detailed description that follows.